High-temperature proportional counter and insulator construction therefor

ABSTRACT

A boron nitride core is coated with polytetrafluorethylene (Teflon) or a similar fluorocarbon resin to form a composite gassealant insulator for use at temperatures up to about 370* C., highly resistant to mechanical deformation or chemical attack. A flow proportional counter incorporating the insulator construction is employed as a detector in gas chromatography.

United States Patent Daniel S. Berry Inventor Ch I [50 1 References Cited lcago, T pp No- 65460] UNITED STATES PA ENTS Filed July 9 19 7 2,567,162 9/1951 Sanders l 17/70 Pammed Sept 28, 1971 3,389,749 6/1968 Towns et al l56/245 Assignee Nuclear-Chicago Corporation OTHER REFERENCES Des Plalnes, Deal, A Radiological Detector for Gas Chromatography,"

HIGH-TEMPERATURE PROPORTIONAL COUNTER AND INSULATOR CONSTRUCTION THEREFOR Analytical Chemistry Vol. 28, No. 12, Dec. 1956.

Primary Examiner-James W. Lawrence Assistant ExaminerDavid OReilly Atl0rneyLeonard G. Nierman 5 Claims, 2 Drawing Figs.

US. (:1 313/93, ABSTRAC A we is with 250/835 polytetrafluorethylene (Teflon) or a similar fluorocarbon 1m. (:1 ..l-l0lj 39/26, resin form P gas-swam insulam' M o 39/28 peratures up to about 370 C., highly resistant to mechanical Field of Search 313 93, 54, deformation of Chemical attack A flow Proportional counter 7; 1 17/010. 10, 161 H, 2l8; 324/33; 250/83.6 FT; incorporating the insulator construction is employed as a de- 161/189 tector 1n gas chromatography.

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36 s IIII;/,:;/I///II 1% S. saw 4 a 4'2 JOIPON .lV/TRIDE' HIGH-TEMPERATURE PROPORTIONAL COUNTER AND INSULATOR CONSTRUCTION THEREFOR The present invention relates to electrical insulator structures for use in high-temperature gas environments, and more particularly to insulator structures for use in flow-type proportional counters such as those employed as detectors in gas chromatography at temperatures of the order of 250 to 370 C., frequently in the presence of highly reactive gaseous components.

A common limitation on the temperature of operation of proportional counters or similar ionization-type radiation detectors lies in the high-temperature characteristics of the insulator structure employed to support the anode wire, particularly where the same insulator also serves as a gastight seal for the counter volume. Among the common insulator materials used for insulating seals in detectors of simple construction are resinous fluorocarbon compounds, notably polytetrafluoroethylene (frequently designated by the trademark Teflon). Such insulators have been found to flow or distort with repeated cycles of heating to the relatively high temperatures encountered in gas chromatography use, which are often beyond their nominal melting point, and thus to lose the intimate contact with a surrounding conductor required to maintain a leakage-impervious seal against the escape of radioactive gas. Special mechanical constructions for compensating for, or ameliorating the effects of, such flow or distortion are disclosed in the copending application of Robert L. Molitor, Ser. No. 441,120, filed Mar. 19, 1965, now U.S. Pat. No. 3,426,232.

It is the principal object of the present invention to provide a high quality electrical insulator which eliminates the necessity of such special mechanical constructions for high-temperature operation of gas-chromatography proportional counters and similar plural-electrode discharge devices permitting employment of simple, and simply assembled and disassembled, insulating-seal constructions such as are used in devices operated at room temperature.

An insulating material sometimes used in radiation detectors designed for high-temperature operation is boron nitride. This substance has the high surface and volume resistivity which are required and is unaffected by temperatures far beyond the melting point of fluorocarbon insulators. With sufficient care, it may be made to effect a good seal. However, it is found that reactive components which are often present in the effluent from a gas-chromatography column gradually impair the characteristics, particularly at the surface, by chemical attack, so that such an insulator provides no satisfactory solution.

The present invention flows from the observation that the rapid loss of integrity of the gas seal which occurs when a material such as Teflon is employed in simple insulating gasseal constructions between metal conductors does not occur when the resin is employed only as a relatively thin coating on a core or body of boron nitride. The compound or composite structure so formed is found to have overall properties chemical, mechanical, and electrical, which are thus far superior in such uses to those of the homogeneous insulators of one or the other of its constituents which have heretofore been employed.

The insulator construction employed in the present invention comprises a body or core of boron nitride with a surface coating of a solid fluorocarbon insulating resin having a melting point higher than 200 C. and of resistivity greater than ohm-cm., of a thickness small compared to thedimensions of the core, preferably from approximately 0.5 mil to approximately lO-mil thickness. The entire core is preferably coated on its exterior surface, but lesser portions of the surface may be so coated if desired; all portions of the boron nitride core which are on the interior of a detector must be coated, however, and it is also particularly desirable to coat the portion at the gas-seal interface, to take advantage of the intimate sealing engagement provided by the surface characteristics of the fluorocarbon solid.

The invention as thus described in its broader aspects may be employed in a variety of detailed structures. In accordance with the patent laws, an embodiment representing a particu larly desirable mode of practice thereof is described below and illustrated in the annexed drawing wherein:

FIG. 1 is a longitudinal sectional view (with a portion shown in elevation) of a gas-chromatograph flow proportional counter made in accordance with the invention; and

FIG. 2 is a fragmentary enlarged sectional view of a portion of an insulator employed in the counter of FIG. 1.

The exemplary proportional counter illustrated is the detector of a gas chromatography system of the type wherein a radioactive component is employed as the indicator of effluence. It employs a metallic tubular cathode 10 which is closed at its cup-shaped lower end II and has a radially extending flange 12 at its upper or open end, gas flow tubes 14 and 16 entering radially through the tubular wall. A gas enclosure and counting volume 17 is defined by the cathode l0, sealed at the top by an insulator cap or plug 18. (It will of course be understood that references herein to upper," lower" and similar relationships are used merely for convenience in referring to the drawing, the illustrated orientation being arbitrarily selected.)

The effluent from a gaschromatography column (not illustrated), which is radioactive gas having a temperature ranging from approximately 250 C. to 370 C., is introduced through the inlet tube 14 to the counting volume 17 and exits through the outlet tube 16. g

The insulator cap 18, in accordance with the invention, has a core 19 of boron nitride. The boron nitride core [9 has a thin coating 20, approximately 3 mils in thickness, of a dispersed resinous fluorocarbon compound on its entire outer surface. The coating is desirably of polytetrafluoroethylene (sold under the trademark Teflon); but may be of polymonochlorotrifluoroethylene (sold under the trademark Kel-F), or another fluorocarbon resin (which term as herein used includes halofluorocarbons and copolymers) of melting point in the same range and resistivity of at least 10' ohm-cm. Where polytetrafluoroethylene (Teflon) is employed, as is preferred, it is desirably coated on the boron nitride core by any of the processes now conventional in applying Teflon coatings to solid substrates such as metals.

The insulator cap 18 comprises lower and upper portions 2] and 22 respectively, and an intermediate radially projecting portion or flange 23, all of circular symmetry. The lowerportion 21 is sized precisely to the inside diameter of the upper end of the cathode 10, to be engaged therewith, and is formed with an internal cylindrical hollow or well 24. The insulator flange 23 is adjacent to the cathode flange 12, a suitable soft metal O-ring 25, disposed about the cylindrical portion 21, being interposed therebet ween. An annular metal plate 26 having a central aperture 28 surrounds the base of the upper portion 22 of the insulator cap 18 and abuts against the upper surface 30 of the insulator flange 22. Bolts 32 and 34 extend through holes 36 and 38 in the plate 26 and are fastened to the cathode flange 12 by engagement with threaded apertures 40 and 42. The tightening of the bolts deforms the soft metal ring 25 at the upper end of the tight interface between the cathode l0 and the lower portion 21 of the insulator, so that the elongated resin-to-cathode gas-seal interface is backed by a further seal 44 between the resin and the deformed ring or gasket.

The insulator cap 18 has a central longitudinal bore 46 extending from the upper end 47 to well 24 in the lower end. A metal bushing 49 extends through the bore 46 and has a head portion 50 at its lower end and a threaded portion 52 at its upper end. A second soft metal O-ring or gasket 54 is disposed about the bushing between the head 50 and the coated surface of the insulator. A nut 58 engages the upper threaded portion 52 and is tightened against the upper surface 47 of the insulator, drawing the bushing head 50 upwardly and deforming the ring 54 against the fluorocarbon coating to make a gastight seal 48 at the lower end of the bore 46.

An axial anode wire 60 is secured at its lower end to an insulator screw 62 which is threadedly engaged to the bottom of the cathode 10. The insulator screw 62 is composed of boron nitride having (except on the threaded portion thereof) a resinous fluorocarbon coating, in similar fashion to the insulator cap 18, and is constructed with a small aperture 64 in its upper portion which communicates with a cylindrical hollow or well 66 in its lower portion. The anode wire 60 passes through the aperture 64 and is fixed to the bottom or tip end of a conical compression spring 68 which is disposed within the well 66 and abuts at its upper end against the upper surface about the aperture 64. The anode wire 60 is maintained taut by the compression of the spring 68; the upper portion of the wire 60 passes through the bushing 49 and extends out through the upper end where it is soldered at 70, which also serves to complete the gas seal. The external portion 72 of the anode wire is connected to a suitable terminal connector (not shown).

The fluorocarbon coating on the surfaces of the insulators l8 and 62 within the counter, and thus exposed to the chromatograph effluent gas, prevents degradation of the boron nitride and maintains a high surface resistivity from the bushing head 50 to the wall of the cathode 10, as well as maintaining a resin-to-metal gas-seal along the long interface and at sealing surfaces 44 and 48 despite the temperature cycling of the device which occurs in each performance of a chromatography analysis. The fluorocarbon coating is sufficiently thin so that the overall insulator has substantially the same resistance to distortion as the core, being merely sufficiently thick to provide the desired surface characteristics. A coating of from about 0.5 mil to about l-mil thickness is desirable, the mentioned thickness of 3 mils being typical.

The embodiment of the invention illustrated in the drawing and described above will readily suggest to persons skilled in the art a large number of variations and modifications, substantially different in appearance and detail, but nevertheless utilizing the basic teachings of the invention. The scope of the protection to be afforded the invention should therefore not be limited by the particular embodiment shown and described, but should be determined in terms of the definitions of the invention set forth in the appended claims, and equivalents thereof.

As is customary, the term melting point" as used herein in connection with fluorocarbon resins such as Teflon refers to the approximate temperature range beyond which softening and pressure-flow are normally considered excessive, being also sometimes designated as the softening point. In the case of Teflon, this is in the neighborhood of 300 C. Where a resin of somewhat lower melting point is employed, the upper limit of the operating range is lowered, but remains above the nominal melting point. The references to resistivity, as is also customary, refer to room-temperature value. In the case of Teflon, this is of the order of 10' ohm-cm. As in the case of any such material, the resistivity is substantially lowered at the highest temperature of permissible use, but here again the restriction to a thin coating greatly reduces the importance of this factor.

What is claimed is:

1. In a plural-electrode discharge device having at least one insulator mutually insulating the electrodes, the insulator having a portion of its surface tightly engaged to form a gas-seal for the interelectrode region and a portion of its surface exposed in the interelectrode region, the improved construction wherein the insulator comprises a boron nitride body having on at least said portions of the surface thereof a coating of a fluorocarbon resin of melting point above 200 C. and of resistivity greater than 10 ohm-cm.

2. The improved construction of claim 1 wherein the pluralelectrode discharge device is a detector for use in gas chromatography having gas inlet and outlet passages connected to the interelectrode region.

3. The improved construction of claim 2 wherein the detector is a flow proportional counter having a tubular metallic cathode and an axial anode wire, the insulator having a fluorocarbon-coated surface portion tightly fitted into the end of the cathode.

4. The improved construction of claim 1 wherein the coating is from 0.5 to 10 mils thickness.

5. The improved construction of claim 4 wherein the coating is of polytetrafluoroethylene. 

2. The improved construction of claim 1 wherein the plural-electrode discharge device is a detector for use in gas chromatography having gas inlet and outlet passages connected to the interelectrode region.
 3. The improved construction of claim 2 wherein the detector is a flow proportional counter having a tubular metallic cathode and an axial anode wire, the insulator having a fluorocarbon-coated surface portion tightly fitted into the end of the cathode.
 4. The improved construction of claim 1 wherein the coating is from 0.5 to 10 mils thickness.
 5. The improved construction of claim 4 wherein the coating is of polytetrafluoroethylene. 